Custom units and calculations, errors in unit conversion.

Hi, I am an Schweitzer Engineering Laboratories, Inc, i don't have the product code. I am trying to do some basic calculations. i was not able to create the custom unit for the thermal resistance, which is used for calculations in estimating the junction temperature in MOSFET.

Theri is some unit conversion problems if I add 1 deg C to 5 deg C the result not 6 deg C instead 279.15 deg C. There are other errors, I really need the help in completing a project, my Email ID is: anuroop_reddy_dasari@selinc.com.